Theme build with yarn

Hello,

In the doc, I don’t find how to use yarn to generate styles from a theme.

Example:
I have themes/myTheme/SyliusShopBundle/private/less/app_fo.less or themes/myTheme/SyliusShopBundle/private/scss/style.scss and I don’t see any change when I run yarn build.

I created a custom class .findme-test-class{} and when I searched it in the entire project I have nothing

The goal is to create a new theme and personalized it with LESS or SCSS. Can you help me? Or tell me where I can find more informations